Urban runoff is killing salmon. Here's how to fix it.

A new study confirms the link between polluted urban water and salmon deaths – and identifies an easy fix to save the coho salmon.

"Our goal with this research is to find practical and inexpensive ways to improve water quality," said Julann Spromberg, a research scientist at NOAA Fisheries' Northwest Fisheries Science Center in Seattle, in a press release. "The salmon are telling us if they work."

Dirty storm runoff, the water that drains from parking lots, highways, and other city spaces, seemed the most likely culprit. To test this idea, the researchers put salmon in water that contained some of the chemicals commonly found in urban runoff.

The fish were fine.

They next put salmon in actual runoff water, collected from a highway in the middle of Seattle. This time, the fish started dying.

The researchers gathered runoff during nine different storm events in 2012 and 2013, all of which had enough toxins to kill adult coho, according to a study in the current Journal of Applied Ecology.

The good news: they could save the salmon if they poured the dirty water through simple soil columns that filtered out the pollutants.

To combat the declining salmon population, the researchers propose introducing new storm water infrastructure technologies, including the soil biofiltration that worked in their experiments.

They have not yet identified which specific pollutant is toxic to coho salmon, or whether it's a particular combination, but they know the soil-treated runoff is safe for the fish.

"Ideally, we would prefer to identify which chemicals are responsible for the coho spawner mortality and implement some form of source control so they would not be released into the environment," says Spromberg. "This can involve either replacing the chemical with a non-toxic alternative, or preventing its release at its use point."